1. Field of the Invention
The present invention relates to threaded connections for use in connecting tubular members and, more particularly, threaded connections used in casing strings and other pipe strings that can be expanded radially to an increased internal diameter.
2. Description of the Prior Art
In U.S. Pat. No. 5,348,095, there is disclosed an apparatus and method for radially expanding well casing after the casing string has been lowered into a well bore. Expansion of the casing string is accomplished by moving an oversized forging tool, or xe2x80x9cpig,xe2x80x9d through the string. The technique permits subsequent strings of casing to be lowered through the previously enlarged casing string sections and thereafter similarly expanded. The result is a well cased by a series of linked sections of casing having substantially the same internal diameters.
Conventional casing strings are made up of a series of individual pipe joints secured together at their ends by threaded connections. Typically, a joint of casing is approximately 40 feet in length and has a threaded male, or pin, connection at one end and a threaded female, or box, connection at the other end. However, the joint may have a pin at each end, successive joints being made up by means of a coupling that has a box at each end to receive the pins on the adjacent joints of coupling. In the other case, the box connection is integrally formed at one end of the casing joint. These integral box connections can be of a larger OD than the OD of the pipe body, or they can have an OD the same size as the OD of the pipe body, the latter case being referred to as a xe2x80x9cflush joint connection.xe2x80x9d
Obviously, one of the problems in expanding casing strings is to ensure that the threaded connections retain their integrity after the expansion process. More particularly, in many cases, it is desired that the casing string be expanded by up to 25% and still maintain a gas-tight seal at the threaded connections. While this can be accomplished with various thread designs, the use of resilient O-rings or other resilient seal rings, it is clearly desirable if a metal-to-metal gas-tight seal can be maintained after the expansion process.
Over and above expandable casing strings, there still remains a need for conventional casing strings that will maintain a metal-to-metal gas-tight seal, even under high bending loads.
It is therefore an object of the present invention to provide a threaded connection for tubular members, such as casing strings.
Another object of the present invention is to provide a threaded connection that concentrates the metal-to-metal sealing between the pin and box connectors at a point of enhanced radial wall thickness.
Still a further object of the present invention is to provide a threaded connection having a gas-tight seal in which the threads can be run out or extend substantially to the axially outermost end of the pin connector and the axially innermost end of the box connector.
Still a further object of the present invention is to provide a threaded connection for tubular members having a gas-tight seal that is maintained upon radially expanding the tubular members by up to 130% of its original diameter.
The above and other objects of the present invention will become apparent from the drawings, the description given herein, and the appended claims.
In accordance with the present invention, there is provided a threaded connection for tubular members that includes a box connector and a pin connector. The box connector has an axially inner, internally threaded section; an axially outer, internally threaded section; and a thread-free section between the inner and outer internally threaded sections. The axially inner and axially outer threaded sections in the box connector form a two-step thread; i.e., a step is formed between the inner internally threaded section and the outer internally threaded section. The pin connector has an axially inner, externally threaded section; an axially outer, externally threaded section; and a thread-free section between the inner and outer externally threaded sections. The threaded sections on the pin connector are also stepped and mate with the threaded sections on the box connector. The mating threads of the pin and box connectors can be of virtually any form. The threaded connection further includes at least one annular relief in the thread-free section of at least one of the pin and box connectors. An axially facing, annularly extending pin torque shoulder is fonned on the pin connector, while an axially facing, annularly extending box torque shoulder is formed in the box connector. A metal-to-metal seal is formed between the thread-free portions of the box connector and the pin connector when the pin torque shoulder and the box torque shoulder are engaged, the annular relief being adjacent and on either side of the metal-to-metal seal.